Switching device

ABSTRACT

A switching device according to the present invention is a switching device for switching a load by on-off control of voltage, and includes an SiC semiconductor layer where a current path is formed by on-control of the voltage, a first electrode arranged to be in contact with the SiC semiconductor layer, and a second electrode arranged to be in contact with the SiC semiconductor layer for conducting with the first electrode due to the formation of the current path, while the first electrode has a variable resistance portion made of a material whose resistance value increases under a prescribed high-temperature condition for limiting current density of overcurrent to not more than a prescribed value when the overcurrent flows to the current path.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching device using SiC.

2. Description of Related Art

In general, a semiconductor power device mainly used for a system suchas a motor control system or a power conversion system in the powerelectronics field is watched with interest.

For example, an SiC semiconductor device is well-known as such a type ofsemiconductor power device (Patent Document 1: Japanese UnexaminedPatent Publication No. 2007-258465).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a switching devicecapable of preventing thermal destruction resulting from a continuousflow of overcurrent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a switching device deviceaccording to a first embodiment of the present invention.

FIG. 2 illustrates an inverter circuit into which the switching deviceis incorporated.

FIGS. 3A and 3B illustrate waveforms of drain current Id upon shortcircuits of devices.

FIG. 4 is a schematic sectional view of a switching device according toa second embodiment of the present invention.

FIG. 5 is a schematic sectional view of a switching device according toa third embodiment of the present invention.

FIG. 6 is a schematic sectional view of a switching device according toa fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A switching device according to the present invention is a switchingdevice for switching a load by on-off control of voltage, and includesan SiC semiconductor layer where a current path is formed by on-controlof the voltage, a first electrode arranged to be in contact with the SiCsemiconductor layer, and a second electrode arranged to be in contactwith the SiC semiconductor layer for conducting with the first electrodedue to the formation of the current path, while the first electrode hasa variable resistance portion made of a material whose resistance valueincreases under a prescribed high-temperature condition for limitingcurrent density of overcurrent to not more than a prescribed value whenthe overcurrent flows to the current path.

According to the structure, the resistance value of the variableresistance portion can be increased when overcurrent flows between thefirst electrode and the second electrode through the SiC semiconductorlayer and the variable resistance portion is brought into ahigh-temperature condition due to heat generation resulting from theovercurrent. Such increase of the resistance value of the variableresistance portion reduces the density of the overcurrent flowingbetween the first electrode and the second electrode. Even if the firstand second electrodes are short-circuited and overcurrent flowstherebetween, therefore, the current density is reduced below theprescribed value as the time elapses. Consequently, the switching devicecan be prevented from thermal destruction resulting from a continuousflow of the overcurrent. In other words, short-circuit resistance of theswitching device can be improved.

Preferably, the variable resistance portion is arranged closer to theSiC semiconductor layer with respect to a center of the first electrodein the thickness direction.

When heat occurs mainly in the SiC semiconductor layer due to theovercurrent, the generated heat can be more quickly transferred to thevariable resistance portion due to the aforementioned structure.Therefore, a period up to a time when the resistance value of thevariable resistance portion starts to increase can be reduced.Consequently, the density of the overcurrent can be reduced in a shortertime.

Preferably, the variable resistance portion is made of a conductivebarium titanate-based compound.

Preferably, the variable resistance portion includes a high-resistanceportion where a rate of increase of the resistance value under theprescribed high-temperature condition is relatively high and alow-resistance portion where the rate of increase is lower than that inthe high-resistance portion, and the high-resistance portion is smallerin thickness than the low-resistance portion, and arranged closer to theSiC semiconductor layer with respect to the low-resistance portion.

According to the structure, the low-resistance portion is provided onthe variable resistance portion, and the high-resistance portion isthinly formed. Even if the variable resistance portion is exposed to ahigh temperature in ordinary use, therefore, the resistance value of thewhole variable resistance portion can be prevented from excessiveincrease. Upon a short circuit, on the other hand, the high-resistanceportion of the variable resistance portion can take the lead inexcellently sufficiently reducing the density of the overcurrent. Inother words, a switching device capable of excellently reducing thedensity of overcurrent while suppressing increase of on-resistance canbe provided.

Preferably, the variable resistance portion limits the density of theovercurrent to not more than 2000 A/cm² within 2 μsec from a time whenthe overcurrent starts flowing to the current path.

The variable resistance portion may be in contact with the SiCsemiconductor layer.

When heat occurs mainly in the SiC semiconductor layer due to theovercurrent, the generated heat can be directly transferred to thevariable resistance portion due to the aforementioned structure.

When the variable resistance portion is selectively in contact with theSiC semiconductor layer, the first electrode may include a contactportion in contact with the SiC semiconductor layer on a portiondifferent from the variable resistance portion.

The first electrode may include a contact portion arranged between thevariable resistance portion and the SiC semiconductor layer to be incontact with the SiC semiconductor layer.

According to the structure, the first electrode can be excellentlybrought into ohmic contact with the SiC semiconductor layer. Preferablyin this case, the thickness of the contact portion is not more than 1μm.

The switching device may have a MIS transistor structure including agate insulating film formed in contact with the SiC semiconductor layerand a gate electrode arranged on the gate insulating film, and thecurrent path may be formed in the vicinity of the interface between theSiC semiconductor layer and the gate insulating film by on-off controlof voltage to the gate electrode.

Even if there is a time lag between start of a short circuit of theswitching device and recovery from the short circuit by off-control of agate (cutoff control of the current path), for example, the density ofthe overcurrent can be sufficiently reduced before the recovery from theshort circuit due to the aforementioned structure.

The MIS transistor structure may include a trench gate structureincluding a gate trench formed in the SiC semiconductor layer so thatthe gate electrode is embedded in the gate trench through the gateinsulating film.

In the case of the trench gate structure, channel resistance andon-resistance tend to be smaller and lower respectively, as comparedwith those of a planar gate structure. The density of overcurrent iseasily increased in a device having low on-resistance, and hence it isdifficult for the device to withstand the overcurrent over a long periodof time, and the device may be thermally destroyed between start of ashort circuit and recovery from the short circuit. When the variableresistance portion is provided on the first electrode as in the presentinvention, therefore, the MIS transistor of the trench gate structurecan also be excellently prevented from thermal destruction.

When the SiC semiconductor layer includes a source layer formed to beexposed on a front surface of the SiC semiconductor layer for partiallyforming a side surface of the gate trench, the first electrode mayinclude a source electrode in contact with the source layer.

When the switching device further includes a source trench formed topass through the source layer from the front surface of the SiCsemiconductor layer, the variable resistance portion is preferablyformed to enter the source trench.

When heat occurs mainly in the SiC semiconductor layer due to theovercurrent, the distance between a main heat generation region and thevariable resistance portion is shortened due to the aforementionedstructure. Consequently, the heat generated in the heat generationregion can be efficiently transferred to the variable resistanceportion.

Embodiments of the present invention are now described in detail withreference to the attached drawings.

FIG. 1 is a schematic sectional view of a switching device according toa first embodiment of the present invention.

A switching device 1 includes a trench gate MISFET (Metal InsulatorSemiconductor Field-Effect Transistor) employing SiC. The switchingdevice 1 includes an SiC substrate 2 and an SiC epitaxial layer 3 formedon the SiC substrate 2. Each of the SiC substrate 2 and the SiCepitaxial layer 3 is of an n-type as a first conductivity type. Morespecifically, the SiC substrate 2 is of an n⁺-type (having aconcentration of 1.0×10¹⁸ cm⁻³ to 1.0×10²¹ cm⁻², for example), and theSiC epitaxial layer 3 is of an n⁻-type (having a concentration of1.0×10¹⁵ cm⁻³ to 1.0×10¹⁷ cm⁻³, for example) lower in concentration thanthe SiC substrate 2. The first embodiment shows the SiC substrate 2 andthe SiC epitaxial layer 3 as examples of the SiC semiconductor layeraccording to the present invention.

According to the first embodiment, the thickness of the SiC substrate 2is 30 μm to 400 μm, for example. On the other hand, the thickness of theSiC epitaxial layer 3 is 3 μm to 100 μm, for example, according to thefirst embodiment.

The SiC epitaxial layer 3 is provided with gate trenches 4. According tothe first embodiment, the gate trenches 4 are formed in a latticedmanner, for example. Alternatively, the gate trenches 4 may be formed ina striped manner or in a honeycomb manner.

According to the first embodiment, the latticed gate trenches 4 are soformed that the SiC epitaxial layer 3 is provided with a plurality ofunit cells 5 on window portions surrounded by the gate trenches 4.

The gate trenches 4 are formed to be U-shaped in section, so that cornerportions 43 where side surfaces 41 and bottom surfaces 42 intersect withone another are curved. The gate trenches 4 are formed by forming theouter shapes thereof by dry-etching the SiC epitaxial layer 3 andthereafter wet-etching inner surfaces thereof, for example. Thus,planarity of the side surfaces 41 of the gate trenches 4 can beimproved. Consequently, collision between electrons can be reduced whencurrent flows along the side surfaces 41 of the gate trenches 41,whereby channel mobility can be increased.

Gate insulating films 6 made of an insulating material such as SiO₂ areformed on the inner surfaces (the side surfaces 41, the bottom surfaces42 and the corner portions 43) of the gate trenches 4, to cover thewhole areas thereof.

Gate electrodes 7 made of a conductive material such as polysilicon areembedded in the gate trenches 4. The gate electrodes 7 are opposed tothe SiC epitaxial layer 3 through the gate insulating films 6.

Source trenches 8 are formed on central portions of the unit cells 5.According to the first embodiment, the source trenches 8 are identicalin depth to the gate trenches 4. The source trenches 8 are also formedto be U-shaped in section similarly to the gate trenches 4, so thatcorner portions 83 where side surfaces 81 and bottom surfaces 82intersect with one another are curved.

Source layers 9, channel layers 10 and a drift layer 11 are formed onthe unit cells 5 successively from a front surface of the SiC epitaxiallayer 3 toward a rear surface thereof, and the layers 9 to 11 are incontact with one another. Thus, the source layers 9 and the drift layer11 are arranged in the vertical direction perpendicular to the frontsurface of the SiC epitaxial layer 3 to be separated from one anotherthrough the channel layers 10, thereby constituting a trench gate MIStransistor structure. The source layers 9 and the drift layer 11 are ofthe n-type as the first conductivity type, and the channel layers 10 areof a p-type as a second conductivity type. More specifically, the sourcelayers 9 are of the n⁺-type (having the concentration of 1.0×10¹⁸ cm⁻³to 1.0×10²¹ cm⁻³, for example), the channel layers 10 are of the p-type(having a concentration of 1.0×10¹⁶ cm⁻³ to 1.0×10¹⁹ cm⁻³, for example),and the drift layer 11 is of the n⁻-type (having the concentration of1.0×10¹⁵ cm⁻³ to 1.0×10¹⁷ cm⁻³, for example) lower in concentration thanthe source layers 9.

The source layers 9 partially form the side surfaces 41 of the gatetrenches 4 and the side surfaces 81 of the source trenches 8. Thechannel layers 10 also partially form the side surfaces 41 of the gatetrenches 4 and the side surfaces 81 of the source trenches 8, similarlyto the source layers 9. The drift layer 11 forms the corner portions 43and the bottom surfaces 42 of the gate trenches 4 as well as the cornerportions 83 and the bottom surfaces 82 of the source trenches 8.

Source breakdown voltage holding layers 12 are formed on the SiCepitaxial layer 3. The source breakdown voltage holding layers 12 are ofthe p type (having the concentration of 1.0×10¹⁶ cm⁻³ to 1.0×10¹⁹ cm⁻³,for example) as the second conductivity type.

The source breakdown voltage holding layers 12 are formed to reach thechannel layers 10 immediately above the corner portions 83 from thebottom surfaces 82 of the source trenches 8 through the corner portions83 thereof.

On the bottom surfaces 82 of the source trenches 8, channel contactlayers 13 are formed on surface layer portions of the source breakdownvoltage holding layers 12. The channel contact layers 13 are of the ptype as the second conductivity type. More specifically, the channelcontact layers 13 are of a p⁺-type (having an impurity concentration of1.0×10¹⁸ cm⁻³ to 2.0×10²¹ cm⁻³, for example).

An interlayer film 14 made of an insulating material such as SiO₂ isformed on the SiC epitaxial layer 3, to cover the gate electrodes 7.

Contact holes 15 larger in diameter than the source trenches 8 areformed in the interlayer film 14. Thus, the whole of the source trenches8 of the unit cells 5 and peripheral edge portions of the sourcetrenches 8 of the SiC epitaxial layer 3 are exposed in the contact holes15. Upper surfaces and side surfaces of the source layers 9 are exposedon the peripheral edge portions.

A source electrode 16 as an example of the first electrode according tothe present invention is formed on the interlayer film 14. The sourceelectrode 16 includes a variable resistance layer 17 and a surfaceelectrode layer 18 stacked successively from the SiC epitaxial layer 3.

According to the first embodiment, the variable resistance layer 17 ismade of a conductive barium titanate-based compound. The bariumtitanate-based compound can be obtained by adding a small amount of analkaline earth metal such as calcium (Ca) or strontium (Sr) or a rareearth metal such as yttrium (Y), neodymium (Nd), samarium (Sm) ordysprosium (Dy) to barium titanate as a main component, for example.Such a conductive barium titanate-based compound is a conductivematerial sensing that the same has been brought into a prescribedhigh-temperature condition and having a resistance value increasingalong with temperature rise.

The variable resistance layer 17 is formed by a single-layer structureof a prescribed thickness, and arranged to follow (to be along) the sidesurfaces 81 and the bottom surfaces 82 of the source trenches 8.According to the first embodiment, the thickness of the variableresistance layer 17 is 0.5 μm to 2 μm, for example. Thus, spaces 19surrounded by the variable resistance layer 17 are formed in the sourcetrenches 8. The variable resistance layer 17 is in contact with thewhole areas of the SiC epitaxial layer 3 exposed from the contact holes15. More specifically, the variable resistance layer 17 is in contactwith the channel contact layers 13, the source breakdown voltage holdinglayers 12, the channel layers 10 and the source layers 9 successivelyfrom the bottoms of the source trenches 8 in the unit cells 5.

According to the first embodiment, the surface electrode layer 18 has astructure (Ti/TiN/Al) obtained by stacking titanium (Ti), titaniumnitride (TiN) and aluminum (Al) successively from the side in contactwith the variable resistance layer 17. The surface electrode layer 18 isarranged to fill up the spaces 19 in the source trenches 8.

A drain electrode 20 as an example of the second electrode according tothe present invention is formed on the rear surface of the SiC substrate2. According to the first embodiment, the drain electrode 20 has astructure (Ti/Ni/Au/Ag) obtained by stacking titanium (Ti), nickel (Ni),gold (Au) and silver (Ag) successively from the SiC substrate 2. Thedrain electrode 20 covers the whole area of the rear surface of the SiCsubstrate 2. In other words, the drain electrode 20 is common to allunit cells 5.

According to the switching device 1, a state of applying no voltage tothe gate electrodes 7 is continued (off-control) so that the p-typechannel layers 10 electrically insulate the n-type source layers 9 andthe n-type drift layer 11 from one another. In other words, no currentpath is formed between a source and a drain, but the switching device 1enters a switch-off state. When voltage exceeding threshold voltage isapplied to the gate electrodes 7 while drain voltage is applied betweenthe source layers 9 and the drift layer 11 (on-control), on the otherhand, current paths (channels) vertically feeding current along the sidesurfaces 41 of the gate trenches 4 are formed on the channel layers 10.This corresponds to a switch-on state.

The switching device 1 can be incorporated into an inverter circuitshown in FIG. 2, for example. FIG. 2 illustrates the inverter circuitinto which the switching device 1 is incorporated. FIGS. 3A and 3Billustrate waveforms of drain current Id upon short circuits of devices.

An inverter circuit 21 is a three-phase inverter circuit connected to athree-phase motor 22 as an example of a load. The inverter circuit 21includes a DC power source 23 and a switch portion 24.

According to the first embodiment, the DC power source 23 is 700 V, forexample. A high-voltage-side wire 25 and a low-voltage-side wire 26 areconnected to a high-voltage side and a low-voltage side of the DC powersource 23 respectively.

The switch portion 24 includes three arms 27 to 29 corresponding to aU-phase 22U, a V-phase 22V and a W-phase 22W of the three-phase motor 22respectively.

The arms 27 to 29 are parallelly connected between the high-voltage-sidewire 25 and the low-voltage-side wire 26. The arms 27 to 29 includehigh-side transistors (the switching device 1) 30H to 32H and low-sidetransistors (the switching device 1) 30L to 32L respectively.Regenerative diodes 33H to 35H and 33L to 35L are parallelly connectedto the transistors 30H to 32H and 30L to 32L respectively in such adirection that forward current flows from the low-voltage side towardthe high-voltage side.

High-side gate drivers 36H to 38H and low-side gate drivers 36L to 38Lare connected to the gates of the transistors 30H to 32H and 30L to 32Lrespectively.

In the inverter circuit 21, AC current can be fed to the three-phasemotor 22 by alternately switching on-off control of the high-sidetransistors 30H to 32H and the low-side transistors 30L to 32L of thearms 27 to 29, i.e., by alternately switching a state where either thehigh-side transistors 30H to 32H or the low-side transistors 30L to 32Lare switched on and either the low-side transistors 30L to 32L or thehigh-side transistors 30H to 32H are switched off. On the other hand,energization to the three-phase motor 22 can be stopped by bringing alltransistors 30H to 32H and 30L to 32L into switch-off states. Thethree-phase motor 22 is switched in this manner.

In a general inverter circuit, noise generated therein may be superposedat a timing when a switching control signal is switched from a low sideto a high side or vice versa, to exceed threshold voltage of atransistor. Referring to the inverter circuit 21 shown in FIG. 2, forexample, a transistor (the low-side transistor 30L, for example) to beoriginally in a switch-off state malfunctions to enter a switch-onstate. At this time, the high-side transistor 30H of the arm 27 is in aswitch-on state, and hence both of the high side and the low side enterswitch-on states to result in a short circuit (arm short).

High voltage (700 V, for example) is applied between thehigh-voltage-side wire 25 and the low-voltage-side wire 26 at the timeof the arm short, and hence overcurrent (the drain current Id) exceedinga rated value (100 A, for example) flows to the malfunctioning low-sidetransistor 30L. In a device (a high on-resistance device) such as an Sidevice having high on-resistance R_(on), resistance increases andcurrent decreases as the time elapses when the device (mainly an SiCepitaxial layer 3) generates heat due to overcurrent as shown in FIG.3A, for example, while such increase of the resistance results from theheat generation of the device. Therefore, the heat generation is notstopped even if the current apparently decreases, and the device isthermally destroyed at a time t₁, for example, when the same is left assuch. In a device (a low on-resistance device) such as an SiC devicehaving low on-resistance R_(on), on the other hand, higher overcurrentflows in a short time as compared with the high on-resistance device,and hence the heat of the device abruptly rises to a high temperature.Therefore, the low on-resistance device is thermally destroyed in ashorter time (t₂<t₁, for example) as compared with the highon-resistance device.

According to the first embodiment, therefore, the low-side gate driver36L senses overcurrent when the overcurrent flows to the switchingdevice 1, to off-control the gate. In relation to the off control of thegate, a short circuit sensing period of about 1 μsec. is provided inorder to prevent a malfunction caused by noise or the like. In otherwords, the low-side gate driver 36L off-controls the gate and reducesgate voltage V_(g) only after sensing that the overcurrent still flowsafter a lapse of the short circuit sensing period. In the lowon-resistance device, however, the time t₂ up to thermal destruction isso extremely short that the switching device 1 cannot be prevented fromthermal destruction if the short circuit sensing period (a short circuitsensing period t₃′, for example) is longer than the time t₂, although noproblem arises when the short circuit sensing period (a short circuitsensing period t₃, for example) is shorter than the time t₂.

In the switching device 1 according to the first embodiment, the sourceelectrode 16 includes the variable resistance layer 17, as shown inFIG. 1. When the variable resistance layer 17 is brought into ahigh-temperature condition due to heat generation in the SiC epitaxiallayer 3 resulting from overcurrent, therefore, the resistance value ofthe variable resistance layer 17 can be increased. Consequently, peakovercurrent can be reduced due to action of the variable resistancelayer 17, as shown by broken lines in FIG. 3B. Thus, abrupt temperatureincrease in the switching device 1 can be prevented, whereby the time upto thermal destruction can be extended from t₂ to t₂′ (>t₂). In otherwords, short-circuit resistance can be improved. While the switchingdevice 1 is thermally destroyed at the time t₂′ if overcurrent is leftas such also in this case, a short circuit sensing period t₃″ can bereliably reduced below the time t₂′ up to thermal destruction due to alonger temporal allowance as compared with a conventional lowon-resistance device. Consequently, the low-side gate driver 36L canoff-control the gate to reduce the gate voltage V_(g) after a lapse ofthe time t₃′ with a sufficient allowance, for turning off current at atime t₄.

Particularly in the case of a trench gate structure such as that of theswitching device 1 according to the first embodiment, channel resistanceand on-resistance tend to be smaller and lower respectively, as comparedwith those of a planar gate structure. The density of overcurrent iseasily increased in a low on-resistance device, and hence it isdifficult for the switching device 1 to withstand the overcurrent over along period of time, and the switching device 1 is easily thermallydestroyed between start of a short circuit and recovery from the shortcircuit. Therefore, the structure of the variable resistance layer 17according to the first embodiment exhibits a particularly excellenteffect of preventing the switching device 1 of the trench gate structurefrom thermal destruction.

According to the first embodiment, further, the variable resistancelayer 17 is arranged to be in contact with the SiC epitaxial layer 3,whereby heat generated in the SiC epitaxial layer 3 due to theovercurrent can be directly transferred to the variable resistance layer17. Therefore, a period up to a time when the resistance value of thevariable resistance layer 17 starts to increase can be reduced.Consequently, the density of the overcurrent can be reduced in a shortertime.

Main heat generation regions of the switching device 1 having the trenchgate structure are positioned immediately under the gate trenches 4where current paths are formed. According to the first embodiment, thevariable resistance layer 17 is arranged to enter the source trenches 8dug down from the front surface of the SiC epitaxial layer 3 and tofollow the side surfaces 81 and the bottom surfaces 82. Thus, thedistance between the variable resistance layer 17 and the main heatgeneration regions is shortened. Consequently, heat generated in theheat generation regions can be efficiently transferred to the variableresistance layer 17.

FIG. 4 is a schematic sectional view of a switching device according toa second embodiment of the present invention. Referring to FIG. 4,portions corresponding to those shown in FIG. 1 are denoted by the samereference numerals.

In a switching device 51 according to the second embodiment, a sourceelectrode 16 further includes a contact layer 39 arranged between avariable resistance layer 17 and an SiC epitaxial layer 3 to be incontact with the SiC epitaxial layer 3.

The contact layer 39 is made of a metal forming ohmic contact betweenthe same and the SiC epitaxial layer 3. According to the secondembodiment, the contact layer 39 is made of Ti/TiN/Al, for example. Thecontact layer 39 has a thickness of not more than 1 μm, and preferablyhas a thickness of 0.5 μm to 1 μm.

According to the switching device 51, the source electrode 16 is incontact with the SiC epitaxial layer 3 on the contact layer 39, wherebythe former can be excellently brought into ohmic contact with thelatter. Further, the switching device 51 can attain effects similar tothose of the switching device 1, as a matter of course.

FIG. 5 is a schematic sectional view of a switching device according toa third embodiment of the present invention. Referring to FIG. 5,portions corresponding to those shown in FIG. 1 are denoted by the samereference numerals.

In a switching device 61 according to the third embodiment, a sourceelectrode 16 includes a variable resistance layer 40 selectively incontact with an SiC epitaxial layer 3, in place of the variableresistance layer 17 in contact with the whole areas of the SiC epitaxiallayer 3 exposed from the contact holes 15.

According to the third embodiment, the variable resistance layer 40 isin contact with peripheral edge portions of source trenches 8 (morespecifically, upper surfaces of source layers 9) exposed from contactholes 15. Thus, the variable resistance layer 40 is not in contact withside surfaces 81 and bottom surfaces 82 of the source trenches 8, but asurface electrode layer 18 filling up the source trenches 8 is arrangedto be in contact with the side surfaces 81 and the bottom surfaces 82.In other words, the surface electrode layer 18 serves as a contactportion in contact with an SiC epitaxial layer 3 according to the thirdembodiment.

The switching device 61 can also attain effects similar to those of theswitching device 1.

The variable resistance layer 40 may alternatively be selectively incontact with upper surfaces and side surfaces of the source layers 9,only the side surfaces 81 of the source trenches 8, or only the bottomsurfaces 82 of the source trenches 8, for example.

FIG. 6 is a schematic sectional view of a switching device according toa fourth embodiment of the present invention. Referring to FIG. 6,portions corresponding to those shown in FIG. 1 are denoted by the samereference numerals.

In a switching device 71 according to the fourth embodiment, a sourceelectrode 16 includes a variable resistance layer 44 of aplurality-layer structure, in place of the variable resistance layer 17of a single-layer structure.

The variable resistance layer 44 includes a high-resistance portion 45where a rate of increase of a resistance value under a prescribedhigh-temperature condition (heat generation resulting from overcurrent,for example) is relatively high and a low-resistance portion 46 wherethe rate of increase is lower than that in the high-resistance portion45.

The high-resistance portion 45 is smaller in thickness than thelow-resistance portion 46, and arranged closer to the SiC epitaxiallayer 3 with respect to the low-resistance portion 46. According to thefourth embodiment, the high-resistance portion 45 is arranged to follow(to be along) side surfaces 81 and bottom surfaces 82 of source trenches8, and the low-resistance portion 46 is arranged on the high-resistanceportion 45. Thus, spaces 47 surrounded by the variable resistance layer44 are formed in the source trenches 8. The high-resistance portion 45is in contact with the whole areas of the SiC epitaxial layer 3 exposedfrom contact holes 15. The thickness of the high-resistance portion 45is 0.1 μm to 0.3 μm, for example, and the thickness of thelow-resistance portion 46 is 0.5 μm to 1.0 μm, for example. A surfaceelectrode layer 18 is arranged to fill up the spaces 47 in the sourcetrenches 8.

According to the switching device 71, the low-resistance portion 46 isprovided on the variable resistance layer 44 and the high-resistanceportion 45 is thinly formed, whereby the resistance value of the wholevariable resistance layer 44 can be prevented from excessive increaseeven if the variable resistance layer 44 is exposed to a hightemperature in ordinary use. Upon a short circuit, on the other hand,the high-resistance portion 45 of the variable resistance layer 44 cantake the lead in excellently sufficiently reducing the density ofovercurrent. In other words, a switching device capable of excellentlyreducing the density of overcurrent while suppressing increase ofon-resistance can be provided. The switching device 71 can also attaineffects similar to those of the switching device 1, as a matter ofcourse.

While four embodiments of the present invention have been described, thepresent invention can be embodied in other ways.

For example, the variable resistance layer 17, 40 or 44 mayalternatively be made of a material other than the conductive bariumtitanate-based compound, so far as the same is a conductive materialsensing that the same has been brought into a prescribedhigh-temperature condition and having a resistance value increasingalong with temperature rise.

The variable resistance layer 17, 40 or 44 may alternatively be providedon the drain electrode 20. In this case, the source electrode 16 mayalso be provided with a variable resistance layer, or only the drainelectrode 20 may be provided with the variable resistance layer 17, 40or 44.

The conductivity types of the semiconductor portions of the switchingdevice 1, 51, 61 or 71 may be inverted. For example, the p- and n-typeportions of the switching device 1 may be replaced with n- and p-typeportions respectively.

A functional element such as a planar gate MIS transistor or aninsulated gate bipolar transistor (IGBT) may alternatively be formed onthe switching device 1, 51, 61 or 71, in place of the trench gate MIStransistor.

The switching device according to the present invention can beincorporated into a power module employed for an inverter circuitconstituting a driving circuit for driving an electric motor utilized asa power source for an electric car (including a hybrid car), an electrictrain, an industrial robot or the like, for example. The switchingdevice according to the present invention can also be incorporated intoa power module employed for an inverter circuit converting powergenerated by a power generator (particularly a private power generator)such as a solar cell or a wind turbine generator to match with powerfrom a commercial power source.

While the present invention has been described in detail by way of theembodiments thereof, it should be understood that these embodiments aremerely illustrative of the technical principles of the present inventionbut not limitative of the invention. The spirit and scope of the presentinvention are to be limited only by the appended claims.

This application corresponds to Japanese Patent Application No.2012-135431 filed with the Japan Patent Office on Jun. 15, 2012, thedisclosure of which is incorporated herein by reference.

What is claimed is:
 1. A switching device for switching a load by on-offcontrol of voltage, comprising: an SiC semiconductor layer where acurrent path is formed by on-control of the voltage; a first electrodearranged to be in contact with the SiC semiconductor layer; and a secondelectrode arranged to be in contact with the SiC semiconductor layer forconducting with the first electrode due to the formation of the currentpath, wherein the first electrode has a variable resistance portion madeof a material whose resistance value increases under a prescribedhigh-temperature condition for limiting current density of overcurrentto not more than a prescribed value when the overcurrent flows to thecurrent path.
 2. The switching device according to claim 1, wherein thevariable resistance portion is arranged closer to the SiC semiconductorlayer with respect to a center of the first electrode in the thicknessdirection.
 3. The switching device according to claim 1, wherein thevariable resistance portion is made of a conductive bariumtitanate-based compound.
 4. The switching device according to claim 1,wherein the variable resistance portion includes a high-resistanceportion where a rate of increase of the resistance value under theprescribed high-temperature condition is relatively high and alow-resistance portion where the rate of increase is lower than that inthe high-resistance portion, and the high-resistance portion is smallerin thickness than the low-resistance portion, and arranged closer to theSiC semiconductor layer with respect to the low-resistance portion. 5.The switching device according to claim 1, wherein the variableresistance portion is for limiting the density of the overcurrent to notmore than 2000 A/cm² within 2 μsec from a time when the overcurrentstarts flowing to the current path.
 6. The switching device according toclaim 1, wherein the variable resistance portion is in contact with theSiC semiconductor layer.
 7. The switching device according to claim 6,wherein the variable resistance portion is selectively in contact withthe SiC semiconductor layer, and the first electrode includes a contactportion in contact with the SiC semiconductor layer on a portiondifferent from the variable resistance portion.
 8. The switching deviceaccording to claim 1, wherein the first electrode includes a contactportion arranged between the variable resistance portion and the SiCsemiconductor layer to be in contact with the SiC semiconductor layer.9. The switching device according to claim 8, wherein the thickness ofthe contact portion is not more than 1 μm.
 10. The switching deviceaccording to claim 1, having a MIS transistor structure including a gateinsulating film formed in contact with the SiC semiconductor layer and agate electrode arranged on the gate insulating film, wherein the currentpath is formed in the vicinity of the interface between the SiCsemiconductor layer and the gate insulating film by on-off control ofvoltage to the gate electrode.
 11. The switching device according toclaim 10, wherein the MIS transistor structure includes a trench gatestructure including a gate trench formed in the SiC semiconductor layerso that the gate electrode is embedded in the gate trench through thegate insulating film.
 12. The switching device according to claim 11,wherein the SiC semiconductor layer includes a source layer formed to beexposed on a front surface of the SiC semiconductor layer for partiallyforming a side surface of the gate trench, and the first electrodeincludes a source electrode in contact with the source layer.
 13. Theswitching device according to claim 12, further comprising a sourcetrench formed to pass through the source layer from the front surface ofthe SiC semiconductor layer, wherein the variable resistance portion isformed to enter the source trench.